Nitrogen trifluoride is widely used in the manufacture of semiconductor materials, in high energy lasers, and in chemical vapor deposition processes. Certain manufacturing processes, such as etching semiconductor material, require a very pure source of NF3 because even small amounts of impurities, especially CF4, can result in the formation of solid residues of carbon or silicon carbide which can cause problems during semiconductor etching operations. To be useful in most electronics manufacturing processes, NF3 must be 99.9% to 99.999% pure.
Nitrogen trifluoride can be manufactured by several processes including electrolysis of melted ammonium acid fluorides, reacting fluorine gas with ammonium bifluoride, and reacting fluorine gas with ammonium cryolite. Most commercial NF3 manufacturing processes involve elemental fluorine (F2) as a reactant. Elemental fluorine is typically manufactured via an electrolytic process that utilizes carbon electrodes. During this process, a small portion of the F2 generated at the carbon electrodes often reacts with the electrodes creating CF4. As a result, the F2 used in NF3 manufacturing processes generally contains at least some CF4 impurities. Additionally, any other carbonaceous impurity in the F2 process or subsequent NF3 process can also react with F2 to create CF4.
U.S. Pat. No. 4,091,081 discloses an NF3 manufacturing process via the reaction 4NH3+3F2→NF3+3NH4F which is run in an ammonium acid fluoride melt. If the starting F2 reactant contains 1 mole % CF4, the product NF3 could contain up to 3 mole % CF4. The actual CF4 content in commercial F2 manufacturing processes varies depending on the cell design, operation, and purpose for which the F2 is generated. Typically, CF4 impurities might vary from 10 ppm to 1% or higher.
Other impurities found in commercially manufactured NF3 include N2, O2, CO2, H2O, CH4, HF, SF6, N2O, and CO.
Unfortunately, it is difficult to separate CF4 from NF3. In fact, the removal of CF4 from NF3 has been described as “practically impossible”. J. Massonne, CHEMIE INGENIEUR TECHNIK, v. 41, N 12, p. 695 (1969). The complexity of separating CF4 from NF3 is due to each compound's low chemical reactivity at normal temperatures, a small difference in the size of their molecules, and a small difference in their boiling points (−128° C. and −129° C. for CF4 and NF3, respectively). GMELIN HANDBOOK, 1986, v. pp. 179-180. The closely related boiling points of NF3 and CF4 make bulk separation of these two compounds by distillation impractical. In addition, the dipole moments and the heats of adsorption of NF3 and CF4 are sufficiently close so that bulk recovery of NF3 from conventional bulk adsorption technologies is not feasible.
Nevertheless, various methods of removing CF4 from NF3 are known in the art, although each has significant disadvantages. For example, U.S. Pat. No. 3,125,425 discloses a process of separating gaseous fluorides by gas chromatography techniques wherein a silica gel having an average pore diameter of 22 Å is mixed with a liquid low-molecular chlorotrifluoroetylene polymer and then used to create phase separation. However, this process suffers from such disadvantages as low efficiency, high consumption of helium or other inert gas (up to 500 liters per liter of NF3 processed), and low effectiveness of the separation when the concentration of impurities is smaller than 1 volume percent. In fact, the purity of the NF3 obtained by this process can not exceed 99 volume percent and, therefore, is inadequate for producing NF3 for the electronics industry.
Other techniques have been developed to obtain NF3 with a purity as high as 99.99%. These methods separate NF3 from CF4 using a zeolite adsorbent that specifically adsorbs NF3. For example, the method disclosed in U.S. Pat. No. 5,069,690 utilizes a gas-solid chromatography technique that involves passing discrete pulses of a mixture of NF3 and CF4 in a continuous flow of an inert carrier gas through a porous bed of an molecular sieve adsorbent consisting of a hydrothermally treated zeolite 5A or chabazite (hydrated calcium aluminum silicate). The molecular sieve in this process kinetically adsorbs NF3 more readily than CF4. Although this process separates NF3 from CF4, to recover the NF3 product, the adsorbent must first be removed from the gas composition containing CF4 and then the NF3 must be extracted from the adsorbent.
Another example of a method utilizing a zeolite molecular sieve is described in U.S. Pat. No. 5,069,887. In this reference, gaseous NF3 containing CF4 is contacted with a crystalline porous synthetic zeolite having an effective pore size of 4.9 Å at a temperature of −50° to 10° C., wherein the NF3 is adsorbed by the zeolite. The remaining gases containing CF4 are subsequently displaced from the adsorbent, and then the NF3 is desorbed from the sieve resulting in a purified NF3 product. The synthetic zeolite of this molecular sieve has a chemical composition represented by the empirical formula Ca6[(AlO2)12(SiO2)12]×H2O and is commercialized under the name “molecular sieve 5A”. This patent specifically limits the described process to using molecular sieve 5A. In fact, this patent states that “it is essential to use molecular sieve 5A as the adsorbent. By using molecular sieve or zeolite of a different class it is difficult to accomplish selective adsorption of only one of NF3 and CF4. If activated carbon, which is a popular adsorbent, is used both NF3 and CF4 are adsorbed.” U.S. Pat. No. 5,069,887, col. 2, lines 9-14
Yet another example of a process utilizing a zeolite molecular sieve to separate NF3 and CF4 is disclosed in Pub. No. US2003/0221556 A1. In this patent, an erionite-type zeolite molecular sieve having an empirical formula (Na,K)9Al9Si27O72×27H2O is disclosed and is used in a process similar to the one described in U.S. Pat. No. 5,069,887.
All of these methods suffer from the severe disadvantage that the adsorbent adsorbs the targeted product (namely NF3) instead of the relatively small amounts of impurities contaminating the NF3. Since the product is adsorbed instead of the impurities, such processes require large amounts of adsorbent relative to the amount of NF3 produced. In addition, large amounts of energy are required to release the bound NF3 from the adsorbent. The low efficiency associated with these purification methods correlates to a high economic cost in obtaining a purified NF3 product.
The present invention overcomes these and other shortcomings of the methods found in the prior art.